Remove the 3 hour warning

Now that we have early warning to notify users if they won't make
their next charge time, the 3 hours warning isn't really necessary
since it will trigger a few hours later anyways. The severe warning
(1 hour/5%) is unchanged. AOSP behavior is unchanged, this only
affects devices which use time based estimates.

Test: sysUI tests pass
Bug: 120030285
Change-Id: Iedbcb7e2a357a0125523bf950818259fb59aefab